Functional Role of BSL1 Subcellular Localization in Brassinosteroid Signaling

The bri1 Suppressor 1 (BSU1) family mediates the brassinosteroid (BR) signal transduction pathway that orchestrates a wide range of developmental and physiological responses in plants. In Arabidopsis, BSU1 family members (BSU1, BSL1, BSL2, and BSL3) enhance BR signaling through hetero- and homo-oligomerization. Interestingly, BSL1 localizes in the cytoplasm whereas the other three homologs occur in both the nucleus and the cytoplasm. However, little is known about whether differential subcellular localization of BSL1 affects oligomerization of BSU1 family members or modulates BR signaling. Here we show that homooligomeric BSL1 forms cytoplasmic puncta and oligomeric combinations between BSU1 family members determine their subcellular localization. We demonstrate that BSL1 has a distinct role in regulating BSL2 and BSL3 through cytoplasmic oligomerization. Overexpression of BSL1 reduced nuclear accumulation of BSL2 and BSL3. Furthermore, mutagenic analysis indicates that nuclear localization of BSL1 promotes BR signaling, suggesting that BSL1 plays a functional role modulating BR signaling through cytoplasmic retention of BSL2 and BSL3.